I haven't found a good way to test major mode highlighting. I end up writing a file that uses all the language features and eyeballing it.
Related Posts
In praise of Tcl, and reflecting on syntax features for a good command shell: https://yosefk.com/blog/i-cant-believe-im-praising-tcl.html
Today I learnt that Emacs 28 shipped a context-menu-mode! https://oylenshpeegul.gitlab.io/blog/posts/20230129/
This seems quintessentially Emacs: deeply hackable, but building UI features in an order very different to the mainstream.
There are *so many* ways that reading a text file can fail.
Maybe it doesn't exist, it's a broken symlink, it's actually a directory, it's not the encoding you expected, or perhaps you just don't have the correct permissions.
Reporting good errors is surprisingly labour intensive.